text:
How can this STOL plane fly so slow?
This plane's ground speed looks slower than a person's walking speed, maybe 10km/h!? It can not be possible that an airflow of 10km/h around wings keep plane in the air.
What is the key for such a slow flight?
a) head wind
b) blast from propeller make additional lift at wings
or
c) maybe propeller upward thrust component help to lift plane in the air

"This plane ground speed looks like slower than man walk, maybe 10km/h!? It can not be possible that an airflow of 10km/h around wings keep plane in the air." An aircraft's ground speed can tell nothing about the speed of the airflow around the wings.

@DeepSpace,Yes I know that is reason why I ask where is the trick! From my intuition airflow speed at wings  must be at least 50km/h to keep 350kg in the air

A headwind nearly sufficient to fly the plane on its own, plus prop wash. For a Dash-7 sized cargo liner that can do that, see an Antonov-2.

Yes to everything but there is also an additional factor you missed. You are right, the lift generated as such slow speeds in such conditions is small. Therefore in order to take off the weight of the entire plane has to be smaller than that! That is the reason for removing significant parts of the fuselage and modifying everything on that plane to get it as light as possible.

Yes, yes and yes:

At that event the winds were 12-15kts.
There is some lift increase from propeller wash impinging on the inboard wing and flap. But most airplanes benefit from this (my own PL-2 has flaps that go all the way under the fuselage and the sink rate difference power on and power on is a lot).
Propeller thrust is contributing to the total lift as soon as the thrust line is tilted above horizontal, as a vertical component is added to the thrust vector. With a slatted wing allowing the wing to operate at up to 25 degrees AOA with the associated deck angle, engine power is starting to contribute a significant part of holding the plane up (if a 300lb thrust force is inclined 25 deg, the vertical thrust or "lift" component is about 130lbs), which is why it plummets the instant the power is removed (to better imagine the effect, just imagine you cankeep pitching the plane's deck angle to 90 degrees, where ALL of the lifting force is coming from the prop).

So, an ultralight like that should have no problem slowing to 21 or 22 kt in that configuration (a Zenith 701 can maintain control down to about that speed although its published stall speed is 30 mph or 26 kts), dropping another knot or two as it gets into ground effect, add in the head wind that was present, and you're down to a ground speed of a person running.
The headwind is kind of "cheating" in terms of absolute performance, but keep in mind that it's a competition between aircraft in those conditions on that day, so the headwind isn't a factor in the relative performance of the competitors, except to the extent it's varying at the time and small variations in wind speed are probably enough to give one plane a victory over others with more or less identical STOL performance.

Still looks imposible to me ,because to have strong propeller wash over wings then plane must accelerate forward,but plane is slow as human walk!How much thrust it has before landing 0:16sec at video?Is max thrust force greater than plane weight?

22kts stall speed,this plane can fly 22kts without engine and head wind??or this include propeller wash?

No power off it's minimum flying speed will be quite a lot higher, on the order of 26-27 kts, where you will run out of tail power to hold the nose up.  Slatted wings on STOL airplanes can't normally be brought to a conventional stall break, but will rather just develop higher and higher sink rates.  I know a guy who was able to tease an actual stall break from his Pegazair which has slats, through some extreme maneuvers, and the reaction was quite violent. Anyway, when you do STOL approaches like that you are totally dependent on power and if the engine quits you are coming down hard.

Thrust line is not tilted so much above horizontal,so it seems that most of lift comes from propeller wash over wings. Maybe they have organize STOL-POWER OFF  COMPETITON!

@member2017 it's travelling over the ground at a walking speed but actually flying about twice as fast through the air.  The winds were 12-15kt as I said.  If you were in a paramotor on that sort of day, which takes off at about 12 kt, you could hover in place if you held it just above the stall speed.

@member2017 at 20-25 degrees the vertical component is not massive but it's not nothing either.  If you go find a vector calculator online and assume the engine makes about 300 lbs of thrust you could work it out.  Maybe 50-75 lbs of lift is coming from the engine (a guess).  Not nothing.

So stall speeds is allways reffer with power ON for every plane?

Published stall speeds are normally power off, and based on a math formula.  Power on stall speeds are normally a bit slower due to the effects I described.  On a normal airplane, maybe 3kt; on a STOL ultralight, maybe a bit more, say 4 or 5 kt.  A slatted STOL aircraft won't really have what you can call a stall break anyway; it just mushes down faster, but there will be some minimum controllable speed you could call the stall speed.  It's always slower with power on.

@John K: Even in a more normal airplane, e.g. a Piper Cherokee, you can use engine power (and nose-up attitude) to fly at a speed which would be a stall with power off.

Just to put the enormous effect of headwind into proportion: tailwind (due to wind shear) has crashed a lot of planes of all sizes. From Wikipedia: "Between 1964 and 1985, wind shear directly caused or contributed to 26 major civil transport aircraft accidents in the U.S. that led to 620 deaths and 200 injuries." Source: https://en.wikipedia.org/wiki/Wind_shear#Impact_on_passenger_aircraft


text:
What is an ATC clearance? Does a VFR flight have to get a ATC clearance?
1)
Do all flights have to get an ATC clearance to fly in controlled airspace such as class C airspace? I know that IFR and VFR flights are provided with ATC service in class C airspace, but I am not sure that receiving ATC service means an ATC clearance must be obtained. And if so, should VFR aircraft also get an ATC clearance like an IFR clearance that is given to a pilot before take-off including clearane limit, route, level, squwak, etc?
2)
What is an ATC clearance? 'ICAO Annex 11' defines an ATC clearance as an "Authorization for an aircraft to proceed under conditions specified by an air traffic control unit." And, also states that an "ATC clearance shall indicate a) aircraft identification as shown in the flight plan, b) clearance limit, c) route of flight, d) level(s) of flight for the entire route or part thereof and changes of levels if required and e) any necessary instructions or information on other matters such as approach or departure manoeuvres, communications and the time of expiry of the clearance."
I think it refers to the IFR clearance given to pilot before take-off.
Does it mean that an ATC clearance always includes all these items such as route or level? If the ATCO instructs a VFR pilot to maintain 5,000ft for separation with IFR, is it just instruction because it doesn't include a clearance limit and route?
In the same vein, is a take-off clearance a type of ATC clearance? It doesn't include some items such as route. But the definition of an ATC clearance states that "The abbreviated term “clearance” may be prefixed by the words “taxi,” “take-off,” “departure,” “en route,” “approach” or “landing” to indicate the particular portion of flight to which the air traffic control clearance relates."

All IFR flights require a clearance in controlled (class A-E) airspace. Clearances aren't available in uncontrolled (class F-G) airspace.
VFR flights in certain airspace (ICAO: class B-D, FAA: class B only) require a clearance and may be referred to as CVFR. This clearance will be much simpler than an IFR one, though.
The purpose of an IFR clearance having a limit, route and altitude is so ATC knows what the plane will do if communication is lost and therefore can maintain proper separation of other aircraft. This detail isn't required for a CVFR clearance because a VFR pilot is required to be able to provide his own separation anyway, so the VFR lost comms procedures are much simpler.
ATC can amend a clearance, in which case they only say the items to be changed, or they can cancel the old clearance and issue a new one from scratch. Which they choose depends on how much is changing and why.
An instruction may authorize a (usually brief) deviation from a clearance, e.g. to avoid traffic or weather. ATC can give instructions to VFR flights without a clearance to amend, though, and this doesn't create a new clearance, so they're something different.

In general, a "clearance" is an authorization for an aircraft to do something.
An "IFR clearance" or "Class Bravo clearance" is one type of clearance, which specifically authorizes an aircraft to enter a certain type of airspace or fly a certain route while receiving defined separation services. See @StephenS's answer for details on that type of clearance. This use of the word is often what is meant by the term "clearance" alone.
But as you point out, ATC can also "clear" aircraft for other actions. In the United States, ATC issues "clearances" for aircraft to take off, land, and execute an approach procedure. Both IFR and VFR aircraft may receive these types of clearances, as they are unrelated to the flight rules status of the aircraft and to any airspace or routing clearance that may or may not have been issued.
ATC also issues taxi instructions, which may be considered "clearances" to taxi on the movement area (controlled surfaces) of an airport but are not referred to as such over the radio.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
What is the difference between a Boeing Line number and a Block number?
We know that every Boeing aircraft has a Line number, Block number, and MSN number. I want to know what what does it mean by this Line Number or Block number. and what are the difference among these terms.

The answer seems to be here.

Here is Boeing's Examples
Block numbers

ZA001 Onwards - B787-8
ZB001 Onwards - B787-9
ZC001 Onwards - B787-10
ZD001 Onwards - rebuild of prototypes
ZE001 Onwards - B787-9 with customizations

Line numbers are the serial number of the aircraft of that aircraft type
Serial Number is the manufacturers serial (basically the number of aircraft they have built up to that aircraft)
Airbus doesn't use the Boeing Line/Serial numbers, they use a different system where the aircraft serial number is in fact the aircraft line number and the total # of the aircraft being built.

Boeing has the following known numbering styles.

MSN (Manufacture serial number) unique to all Boeing aircraft, no repeats

l/n (Line Number) unique in most case to Boeing model. Reset to 1, with 737NG

Customer Code (dropped since on 777,767,737,747) never applied to 787 and 737 MAX, and will not be applied to the 777X either..

Variable Number (treated like L/n but with single alpha prefix)
P = 737, Y = 737NG, R = 747, N = 757, V = 767, W = 777, Z = 787.

Variable Code
As listed by David Cummings is the example best known on the 787 series as used in some usage with testing prototypes with their FAA Reg Number/Tail numbers.
Treated like blocks in production for certain airlines hence the block reference or label also known as tabulation number.
Some number are skipped in the above sequence listed, when can, but no hard and fast rules apply to this process.

I am actively searching for the complete list of Variable Code / Numbers for most 7X7 series aircraft. (have most 747, all 787)

Hi Jets - you should contact me I have most f not all boeings stuff available

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What short-term alternatives do airlines have to replace their grounded Boeing MAX 8s?
The MAX 8 fleet is grounded: what are the other practical aircraft platform options available to AA, SWA or Continental that ensures that ticketed summer 2019 passengers arrive at their destinations safely?

The MAX 8 will likely be flying again by the end of July or early August.

Is passenger tickling a new option in MAX-8 entertainment systems? ;-)

Let’s hope so, Juan.  Boeing promised early April and it just didn’t happen.

Ticketed: gotta love the spellcheck.  :)

I don't think Continental will have to worry about this.

Boeing didn't promise it this time. The FAA did.

Airlines really don't have a lot of options to carry through with the flights they planned with the grounded airplanes.
Older models of the 737 aren't much of an option. Most of the MAXes weren't intended to replace retiring aircraft, and even if they were, various costs of bringing the older aircraft into service for just a few months would be too expensive to make sense in the long run. The A320 would be the most direct replacement, but even then the situation is similar. New aircraft are ordered years in advance, so they can't just go buy an A320neo and get it right now.
It's possible to use a larger aircraft like the 757, but they would have to be pulled off another route and would burn more fuel.
The summer is peak travel season, so there just isn't any spare capacity out there that's economical to bring on board on short notice for just a few months. Even if they did, once the hundreds of MAXes return to service, the excess capacity is a liability.
Aircraft are a very expensive asset. Fleet needs are planned well in advance to have only as many as they absolutely need. Even an unexpected increase in maintenance time causes cancellations. The downside to this is that they're in trouble if a large disruption hits. The cheapest option is to just cancel a lot of the flights.

There would have been a scramble to lease airplanes, any airplanes, that can complete some of the trips, but the pool of available airplanes would have dried up pretty quickly.  Boeing would have been footing the bill for leasing, as well as canx costs, depending on how solid the AD clause in the purchase contract was, so things like extra fuel burn would be more of a problem for Boeing.


text:
How should airline pilots find out about military activity along their route?
What sources of information should a commercial pilot review before takeoff to become aware of military activity that might pose a risk to their flight?

I'm not sure what you're asking. Are you asking about awareness of routine military training within a country (which one?), or international conflicts, or something else?

Your title asks about military but your question is commercial.

@Pondlife  Thanks for the feedback: an attempt to clarify is provided.

Military operations are conducted in certain airspace blocks that should be well marked on your charts.  If you are IFR your route should avoid such areas, if you are VFR, hope you know what you are doing.

All countries chart Special Use Airspace (with various classifications such as Prohibited, Restricted, Danger, Warning, Alert, etc.) designated for military training, defense or other national security purposes, along with the hours it is active (if not full time). Many national and international authorities also publish lists/maps of conflict zones and risk levels.
When planning a flight, airline dispatchers will generally avoid SUA and conflict zones. This includes even SUA expected to be inactive at flight time because delays before departure or en route could change that, so it's simpler to just use a route that is valid at all times, which is generally easy to do by following charted airways. However, special routes to avoid severe weather, which are inherently time-sensitive anyway, may go through inactive SUA.
Different airlines will have different policies on conflict zones and how much risk is too much; some knowingly fly into conflict zones to cash in on desperate travelers with no other options, though they usually stop doing that after one or two of their planes get shot down.
The pilot will review the flight plan before departure and contact the dispatcher with any questions, which a route through SUA or a conflict zone would likely generate, and won't accept it until they are satisfied. However, since they spend all day flying planes rather than staying up to date on military conflicts around the world, they are often at the mercy of dispatchers (and airline policy) when it comes to rapidly changing conflict zones.


text:
How often does an airline pilot use the rudder pedals during flight?
How often does an airline pilot use the rudder pedals during flight? Is it only during takeoff and landing?
I am interested to know the rudder pedal usage time in minutes per average short range flight (A320, Domestic flights, ~1-2 hr flight duration). When on autopilot, the rudder control will be automatic. There are even automatic rudder trim features for most of the commercial aircraft.

I'm guessing that by "brake pedal unit", you mean the rudder pedal (which often has brake pedals attached to it)? As far as I know, it's never called the brake pedal unit; you may want to edit this question and change "brake pedal unit" to "rudder".

I edited your question to try to make it a little clearer, but if I got it wrong you can just roll back my changes or edit it more yourself.

Transport category aircraft have a yaw damper that will maintain coordinated flight when the aircraft is in the air unless the flying pilot overrides it with the rudder. It operates even if the autopilot is not in use. The usual procedure is that you take your feet off the rudder pedals as soon as you have lifted off and don't put them back on the rudder pedals until on short final.
The pilot will possibly use the rudder shortly before or going into the landing flare to slip into a crosswind and/or to align the aircraft with the runway if they were flying a crabbed final. However, with most if not all transport category there's nothing wrong with touching down in a crab correctly being flown to correct for a crosswind if desired.
So whether a short range light or a long haul flight, the amount of time in the air that the pilot needs their feet on the rudder pedals is minimal and is in seconds, at most a minute or so.
An exception, of course, is if there is an engine failure, in which case the pilot will use the rudder to counteract the adverse yaw from the asymmetrical thrust.

Note: this answer was written before the question was edited to only ask about airliners.
The answer varies a lot depending on the type of plane, but ideally, the pilot will manually apply rudder only rarely.
Rudder will be used to coordinate turns and to slip for crosswinds on landing (if not crabbing). Other than that, the rudder should be trimmed so the pilot doesn't need to manually apply it for straight and level flight.
Some small aircraft lack variable rudder trim; if the trim is not correct, they may need to apply rudder throughout the entire flight--and hopefully fix the trim tab before the next flight.
Single-engine planes will need some right rudder input during full power (e.g. takeoff and climb) due to the left-turning tendency. Multi-engine planes don't have that problem, but rudder is very important when dealing with a failed engine--which should be rare outside training.
A three-axie autopilot will apply rudder as needed to keep the plane coordinated. Small planes may have a one- or two-axis autopilot that requires manual coordination. And, of course, autopilot is not used for the entire flight; the pilot will have to use rudder manually during hand-flown portions such as takeoff and landing.

At low speeds any airplane needs the rudder to maintain direction and the ailerons to keep the wings level.  That's why you see the pilot jockeying the yoke of an airliner during landing videos.  I've not flown a 747 but did manage to finagle some time in a simulator once a long time ago.  No expert but I found the technique worked there as well as it worked in the Aztecs and Bonanzas I did fly.  I used rudders to maintain my heading during cruise and to coordinate turns.  I also found many students did not grasp this though they had private licenses (or more) and even several hundred hours.  Like any control, rudders need to be learned.  I discovered when I first flew a Piper Cub many many moons ago that I could stick a bank on the airplane and if I didn't coordinate the turn the Cub would not turn so easily.   In fact, it seemed to fly straight ahead with a bank on it!  That taught me why you use rudders!  I had been spoiled by the Cessna 150s I learned on (no criticism of the designers of frise-differential ailerons).  An hour of practice banking a plane while trying to keep the nose at one point on the horizon may seem like a pointless exercise but it does ensure you can coordinate turns and tune you into the correlation of ailerons, rudders and elevators.  The correct answer to the question would be "As many times as he needs to to coordinate his flight"  I'm not being facetious.  It depends on the conditions of flight as much as the airplane.

I am not sure about usage during autopilot, but without autopilot, you always keep your feet on the rudder pedals. Depending on the number of engine, it can have an engine configuration where it bias towards one side (as with one engine in front, for say a Cessna 172). To make a coordinated turn, one need the rudder, and to counteract the wind/drift to keep a straight course. So, pretty much always. There are no rudder trim to my knowledge, as the wind is never constant all along a mission, even if following a straight heading the whole time.

You are correct that you need rudder in a light aircraft to make a coordinated turn when hand flying. However, you are NOT correct in believing that the rudder is used to counteract the wind/drift to keep a straight course. That is accomplished by flying a straight and level heading that will correct for that part of the wind vector blowing you off course. Thus you will not "pretty much always" be using the rudder.

C172 has a rudder trim tab, but it can't be adjusted in flight. Larger aircraft, like even the C182, do have a pilot rudder trim control. You generally only need to adjust it when changing power levels, to account for variations in left turning tendency. You would still use the rudder pedals to coordinate turns, not trim. Correcting for wind is generally done with a crab at cruise; you would only slip for landing--and some prefer crabbing for that too.

Re "You would still use the rudder pedals to coordinate turns, not trim. Correcting for wind is generally done with a crab at cruise; you would only slip for landing--and some prefer crabbing for that too." -- it seems unlikely that OP of this answer appreciates the difference between a crab and a slip, in terms of required rudder usage.


text:
Why is 3000 psi the most common hydraulic pressure used in commercial aircraft?
As per the article on this website (from 2002), most of the aircraft use 3000 psi as hydraulic pressure:

Since the 1970s, military aircraft have been using 5000 psi hydraulic
  systems, which use lighter-weight components to actuate the flight
  control surfaces. Passenger aircraft have been relegated to 3000-psi
  hydraulic systems (with the exception of the Concorde, which features
  a 4000 psi system).

What is the technical rationale behind using 3000 psi? Is there any historical reason or regulation to use 3000 psi only?

The linked article directly answers the question

"Above 3000 psi, using aluminum as a pressure vessel becomes risky," says Galloway. "5000 psi systems almost exclusively rely on steel or titanium. To minimize stresses, the sizes of passages can be reduced. Computational fluid dynamics is also used to minimize pressure drop in critical areas."

3000 PSI was the best you could get with technology in the 1950s, when standardization occurred. Without economical titanium tubing and reservoirs, using steel would incur a weight tradeoff that doesn't make sense.
The A380 was a trigger to the development of 5000 PSI civilian systems because of both the high demands and the long run distances, both due to the size of the plane. I read somewhere that they took a look at the military 5000 PSI systems and realized it would never stand up to commercial use, so little technology was carried over.
Every new design large transport from the A380 onwards (787, A350) is 5000 PSI.

The civil aviation industry likes to use what is proven and trustworthy, and will continue implementing designs that a are mature and safe. The design safety is 1 : 10$^9$ flight hour, and every time a change is made it takes a truly long time before the reliability and fault tolerance is proven.
So if 3000 PSI systems do the job on civil airliners, they will continue to be used in new design aeroplanes, unless there is a compelling reason to implement a fundamental change, which will result in deep R&D into applicability, reliability etc. Is there any fault condition that can impair passenger safety.
There is really old technology on board of civil airliners, for instance Intel 80186 and Motorola 68020 processors are still being mounted in new aircraft. A ripe old processor design sold 30 years ago, the stone age of the digital era.
Military aircraft are different. Safety is not the overriding factor, but mission capability is. Yes the pilot must be brought home if the plane comes down, but above all it must do its job competitively. So in military aircraft there is a drive towards modernisation, while in civil aviation there is a drive to not change anything that is working reliably, unless a specific problem must be solved.
3000 PSI was what the first hydraulic systems used for operating pressure, the initial hiccups were fixed, and there it remained - until there came an aircraft that was so huge that the line losses simply became too big. It had to be done. Fortunately the system parts for 5,000 PSI were already available, from the military suppliers. That is where a lot of new designs mature.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
What portion of an Instrument Proficiency Check must be in the airplane?
I was asked this question by the Chief Instructor at my Part 141 school. I've looked all over and I can't seem to find an answer to this question. We often use the AATD which is a Redbird full motion but I've not heard anything about a specific portion being in the airplane whether or not you did it all in a sim, or a mixture of simulator and airplane. Can anyone elaborate on this? Thank you in advance.

This is explained in the Instrument Rating ACS (page A-18):

Instrument Proficiency Check
If a person fails to meet the experience requirements of section 61.57(c), a pilot may only
  establish instrument currency through an instrument proficiency check
  as described in section 61.57(d). An FSTD may be used as part of an
  approved curriculum to accomplish all or portions of this check. If
  specified in its LOA, an AATD may be used to complete most of the
  required Tasks. However, the circling approach, the landing Task, and
  the multiengine airplane Tasks must be accomplished in an aircraft or
  FFS (Level B, C, or D).A BATD cannot be used for an instrument
  proficiency check. Please see the Task Table in Appendix 5 for
  additional information. (Appendix 5, Instrument Proficiency Check
  Table).

To summarize:

If the simulator is an approved FFS, you can do the whole IPC in it
If the simulator is an approved AATD, you can do everything except circling, landing and multi-engine tasks
Otherwise, you need to use an aircraft

Specific info available at the Redbird website


text:
Are there any known maintenance issues with older Mooney wooden wings?
Are there any maintenance issues or ADs for the older Mooneys with wooden wings? I'm looking at a 1958 M20A (always hangared) and I'd like to know if there are specific points to check.

Welcome to aviation.SE! If you're not familiar with the site you might like to take the [tour].

You should have an A&P inspect the aircraft/logbooks that is experienced with wooden aircraft, and preferably, wooden Mooneys. It isn't like buying a car where you can walk around and kick the tires, a pre-buy inspection on an aircraft should be a lot like an annual where panels are opened up and is thoroughly inspected.

Get a Mooney specialist with lots of experience on the wood wing models. Be ABSOLUTELY certain that the tail has been metalized. Check out www.mooneyspace.com and ask those guys for their advice. Read this: http://www.acmp.com/blog/buyers-beware-mooney-m-20a-and-bellanca-cruiser.html

You may want to look around here or here or even here but the short answer is they are a wooden wing airplane that comes with all the issues a wooden wing airplane does. One of the major things to look out for is the M20A's that had a wooden tail, those should generally be passed up on. The later planes should require some inspection of the joints where the tail meets the rest of the airframe as it is a stress point (if memory serves). For what its worth you are better off with a metal mooney. With a wood plane you need to inspect for rot the way you would for corosion on a metal plane.
Like any wood aircarft the M20A's can suffer from delamination which would need to be checked for. unfortunatly the list of people that still work on wooden planes is dwindling with time so it may be hard to repair this. It would be well worth the effort to seek out a mechanic that has experience with the A model and can work on a PPI for you.
You can find all the AD's for the M20A here.

See FAA AD 86-196-10 re: Wood Tail Mooneys (M20 and M20A)
Don't buy a wood-tail Mooney, unless you don't mind falling from the sky when it rots off. Very bad design. Later all-metal Mooneys have good safety record. But wood Mooneys -- wood wing and/or tail -- are just the opposite.
Most of Al Mooney's designs (Alexander Bullet, Monosport G / Dart, Culver Cadet, Culver V, Mooney Mite, M20) seem to have poor crash-and-burn record (several of the prototypes crashed -- some fatally -- even before production). Ralph Harmon (Beech Bonanza designer) metalized the M20 into the M20B, fixing its fatal flaws.

Quoting the relevant section from the link would is typically recommended.

I have some experience with the M20F aircraft; however this is an all metal Mooney.
This gentleman may have some good advice for pre-purchase inspections to conduct if you are hunting for a M20A.
http://www.n6030x.com/

This would be better as a comment since it does not seem to add anything to answer the question.


text:
Why does an Avro RJ85 have four engines?
The British-designed Avro RJ85 is a short-haul airplane with four jet engines.  What is the purpose of four engines for such a small aircraft?

It was originally the BAe 146 and was designed for operating out of city airports such as London City Airport.
The main reason is for noise abatement. Back in the 80s, four high-bypass turbofans made a lot less noise than two contemporary engines.
Climb performance is also fantastic.  it can climb safely from most airports using much less than full power which helps keep the noise down and provides high safety margins when operating near built-up areas, for example if an engine fails on take-off.

I have to say it works too, I'm under one of London City's departure paths and I really don't notice the BA-146's much.

Another example of the same idea using turboprops is the DeHavilland Canada  DHC-7 "Dash 7"

The reason it has four engines is that it was originally designed to serve remote airfields- the four engines allowed for better short field performance and redundant systems. From Flightglobal.com:

BAE's ABJ vice-president Stewart Cordner ... unusual design aspects of the 146/RJ, including the fact that the type is equipped with four engines and has an airliner-sized cabin.
"I want to bust this myth on the four engines," he says. "It's a unique aircraft design because it's got short-field performance and redundant systems, as it was originally designed to serve remote airfields.

The aircraft is also suited for operations in noise sensitive city airports- it has no thrust reversers, with large air-brakes and full width wing spoilers. These were initially designed with the idea of using the aircraft as a 'feederliner' and regional airliner.

Cordner believes the four-engined146/RJ is also more suited for operations at noise-sensitive airports ... as it is equipped with large wheel brakes and no thrust reversers. Installation of an extra battery allows the first engine to be started electrically without ground power if the auxiliary power unit is unserviceable, which is attractive to operators serving remote locations.

So, basically, the aircraft has four engines due to to its intended use- in remote airfields. From Flightglobal archive:

Perhaps the most frequently asked question about the 146 layout is, why four engines instead of just two? A cynic might reply that  the  146 needs  roughly 26,8001b of thrust, and that the only way to achieve this with  current turbofans is to use four Avco Lycoming ALF502R-3s, But this is   only a partial answer. The main advantage of four engines is in hot and    high performance from short runways—conditions typically experienced by  feederliners.
In any multi-engined aircraft, performance is dictated by the aircraft's behaviour after an engine has failed. It does not matter whether an   air-craft has lost one of two engines on take-off, or one out of four—it must still be able to clear an obstacle at the end of the runway and achieve a safe climb rate. The BAe 146's four turbofans  give it a greater reserve of power, allowing it to  fly in and out of difficult airfields.
Other advantages claimed for four engines include the ability to make  three-engine ferry flights without passengers, and the retention of  more   than half the aircraft's electrical and hydraulic power when an engine  fails. Three engine ferry flights would allow an airline to fly its  146s back to base for an engine change, which is much simpler and cheaper than doing it in the field.

Actually, a variant was produced with two engines, but production stopped after only two.

Simon's answer, but with a lot more detail. Very nice.

Originally the aircraft was designed with two engines. The problem was that there were not engines with enough power and small size to put it on the aircraft. The only one engine suitable was the Alf 502,  but it was not power enough so they had to install  4 in stead of 2. Also they had to redesign the aircraft because in the original model the engines were located on the rear part of the fuselage( like a MD87 or CRJ)

Welcome to the site. Do you have any references for this information?


text:
Are there any regulations on fueling aircraft inside hangars?
My FBO routinely refuels aircraft with 100LL while they're parked inside hangars. I have complained about this being a dangerous practice. My previous FBO required aircraft to be at least 10ft outside the hangar for refueling. Are there any regulations or advice on this?

Your previous FBO is correct. According to FAA Advisory Circular 150/5230- 4B Aircraft Fuel Storage, Handling, Training and Dispensing on Airports:

Standards The FAA uses the standards contained in the most recent edition of National Fire Prevention Association (NFPA) 407, Standard for aircraft Fuel Servicing. NFPA 407 provides a standard for the storage and delivery of aviation fuel in an airport environment

NFPA 407, on the other hand requires outdoor fueling.

5.10 Aircraft Fuel Servicing Locations.
5.10.1 Aircraft fuel servicing hall be performed outdoors...
5.10.2* Aircraft being fueled shall be positioned so that aircraft fuel system vents or fuel tank openings are not closer than 7.6 m (25 ft) to any terminal building, hangar, service building, or enclosed passenger concourse other than a loading walkway.

A revision of the same standard also looks the same, with some differences in wording. So, the aircraft fueling should be done outdoors. Its doubly important if the hangar/airport where your aircraft is located also follows NFPA 407.

I have never heard of any regulation like that. However this powerpoint may be of interest to you, and 14 CFR 139.321 Covers the topic to an extent,

(b) Each certificate holder must establish and maintain standards
  authorized by the Administrator for protecting against fire and
  explosions in storing, dispensing, and otherwise handling fuel (other
  than articles and materials that are, or are intended to be, aircraft
  cargo) on the airport. These standards must cover facilities,
  procedures, and personnel training and must address at least the
  following:
(1) Bonding.
(2) Public protection.
(3) Control of access to storage areas.
(4) Fire safety in fuel farm and storage areas.
(5) Fire safety in mobile fuelers, fueling pits, and fueling cabinets.
(6) Training of fueling personnel in fire safety in accordance with
  paragraph (e) of this section. Such training at Class III airports
  must be completed within 12 consecutive calendar months after June 9,
  2004.
(7) The fire code of the public body having jurisdiction over the
  airport.

This AC on the topic also does not really talk about fueling indoors or otherwise lend any advice about it.
It may just be that your local airport procedures called for not fueling in hangers. There is an interesting debate on the topic here. That discussion would elude to the fact that there are problems that can arise and some cite cases of it causing an issue. But there are no hard regulations about fueling indoors.


text:
Was the 737-300's wing an early supercritical wing?
I know that the 737 NG borrows the basic wing design of the 777, but I have also heard that BAC 4xx airfoil on the 737 classic (300/400/500) was an early supercritical aerofoil design?
Is this true?

Supercritical yes, but early? Not really.

BACXXX airfoil plot, shamelessly copied from airfoiltools.com.
The plot of the BACXXX above shows typical contour characteristics of supercritical airfoils:

blunt leading edge
very little curvature on the top surface at mid-chord
higher curvature at the bottom, resulting in negative camber around mid-chord
rear loading: Higher camber at the rear, thinner part of the airfoil,

however, this airfoil does not take those techniques to their extreme. It was a redesign on the original 737 wing, extended by 4.4% of chord at the leading edge and overall a compromise between high commonality with the earlier design and improved aerodynamics.
At the time when the "classic" series of the 737 was designed in the late Seventies/early Eighties, supercritical airfoils were already firmly established. They were first researched by K. A. Kawalki in Germany in 1940, who arrived at identical shapes as R. T. Whitcomb at NASA a quarter of a century later. The first operational supercritical airfoils, at the time called rooftop due to the roof-like shape of the upper surface pressure distribution, were employed by Vickers in the VC-10. This remarkable aircraft is to this day the fastest airliner with a maximum cruise Mach number of 0.89 and flew first in 1962.
Boeing became aware of supercritical airfoils only after the original 737 was designed, so the redesigned wing of the classic series carries some of the baggage carried over from the earlier design.


text:
Why would both wingtip lights be red?
I live under Chicago's airspace. One evening I saw a plane approaching Chicago with both wingtip lights red and the front landing lights flashing. I've never seen that before, why would that happen?

Welcome to aviation.SE! Are you completely sure about what you saw? The wingtip lights are required by law to be red and green, but the beacon (anticollision) lights,are usually red too. Is it possible that you were looking at an angle, so that you might have seen one wingtip plus one beacon light?

Which part of Chicago was this in? (Can you tell us which airport it was approaching?)

You might have been seeing a general aviation aircraft from an orientation similar to the one pictured.

You'd see the left navigation light which would be red.
The rotating beacon would also be red, but if the airplane was positioned like this and the beacon was the rotating, not blinking type, you might just see two mostly constant red lights. Being that it was dark you might mistake the second red light for the navigation light on the other wing.
It is becoming more common on general aviation aircraft to have a pulse option on the landing lights. I flew for a 135 operation for a while under which our contract required this to be installed and operational for all our aircraft.
This seems to be the simplest explanation for what you are describing. If we consider Occam'z Razor it is likely to be what you saw on the occasion you described.

an orientation similar to the one pictured You mean it was flying in mirror fashion?

Or maybe on a Mooney you have to put the tail number on backwards too :D

@TomMcW: Well (after a search online) I believe this is to facilitate reading in the rear mirror of the preceding aircraft.

Yea I horizontally flipped the image. Laughed pretty hard at the comments. ;)


text:
Why do airlines retire the flight number after a crash?
Perhaps it seems obvious, but I couldn't find a plausible reason why they do so other than that it evokes negative emotions among passengers and crew. Could there be other reasons to do so?
For example, flight number MH370 was retired as a "mark of respect" for the passengers and crew.
And out of interest, is there any case where an airline company reused a retired number for any reason or in certain circumstances?

My understanding is that flight numbers are retired only if there were fatalities. I could be wrong though.

At the risk of stating the cold hard capitalist view, companies don't truly retire or reuse flight numbers on moral grounds but rather on economic grounds. As it happens, people are (rationally) still willing to fly the same airline and same flight, but (irrationally) less willing to buy a ticket for a recently-crashed flight number, so companies that have a (fiduciary) duty to maximize value for shareholders will (rationally) renumber the flight. In almost all cost-benefit analyzes, a renumbering costs less than expected sales.

consider if you Google a flight number you can see the flight status, imagine if the number was from a past crash, you might for a moment think the one you're looking up had crashed

That's disturbing.  If I were to pick someone up on an AC 797 flight, and I Googled it, this is not the screen I want to be looking at.

IMHO this isn't the main reason (like other answers mention and I also believe - it's to do with emotions), but it's a valid point I wouldn't think of.

Flight numbers were retired after crashes long before Google ever existed.  While this is part of a good reason to do it now, it isn't the whole reason why it has generally been done in the past.

@J...  Fair enough "imagine you're Lycos'ing a flight number and..."

I'd say it's a modern example of the emotional reasoning

@J... Imagine your friend told you s/he was flying on AC797 and then another friend told you AC797 crashed and 29 people died.

Retiring flight numbers after crash is mainly done to prevent the flight evoking negative emotions among future passengers. You don't want the flight number to conjure up images of crash while booking tickets, especially in when you type the flight number and google shows up the wreckage just below the flight data.
Also, it would be quite insensitive, with friends and relatives being reminded of their loved ones every time they hear about the flight.
However, it is not necessary that the airlines have to retire the flight numbers because of a crash. A number of them are being used, a few examples being:

Air Canada Flight 797
Delta Airlines Flight 723
Air Asiana Flight 214

Seems a bit over the top to me. It's just a number, and can be assigned to any plane/flight. "Flight 797 is bad! Flight 798 is good!"... :/

@SnakeDoc that seems rather egocentric.  I'm sure those who have lost a loved one in a crash would disagree, for starters.

@moodboom Do we retire car models when loved ones die in a crash?

@SnakeDoc good point.  Although maybe in the case of cars like the Pinto and Yugo, we should.  :-)

You answered your own question, they retire numbers because of the emotional context. Some people think that using such a number again shows a lack of sensitivity to the dead and their families, others might think that getting on a flight with the same number would be bad luck. It's irrational, but many people would feel uncomfortable getting on a flight with the same number.

It's also done for calming passenger superstitions about such things as well.  Kind of the same reason that many skyscrapers do not have or list a 13th floor or nobody would board a flight listed as "United 666" or that left handed people are possessed by demons, etc.  You would like to believe modern people would not be influenced by such nonsense, yet how many people did you see reading horoscopes on the train on your way to work this morning?

Anecdotal comment. Finnair flight 666 to Helsinki (HEL) departs every day from Copenhagen. Flies on Friday the 13th as well.

Try doing that same flight in the US or some other nation which is very religious - you'll quickly meet stiff resistance.  I've met people who won't pay a bill if it totals $6.66.

I flew AY666 in row 13 (seat F, to get another 6) on a Friday 13th, just for the HEL of it. And just to give Fate the middle finger, I bought a Lego airport fire engine in the departure lounge at CPH. Listening to conversations on board, several people had chosen the flight deliberately; one woman had had three members of her family diagnosed with cancer in six months and was hoping to "break the curse".

@J.Hougaard "Finnair Flight 666 to HEL makes final Friday 13th departure.". They seem to have renamed the flight in 2017

I am a retired airline employee, the reason for retiring a flight number is for the respect of any fatalities. No other secret reason. (At least the “original” Airlines) it was ALL ABOUT RESPECT AND NOTHING ELSE.


text:
Which jet engines have the highest thermal efficiency?
Some automakers (Mercedes-Benz, Toyota) have recently been making news about the improved thermal efficiency of their engines, stating that they reached the 50% mark. I was wondering if it was the same for airplanes.
I think I'm pretty sure the aviation industry is well beyond the 50% mark due to the high compression ratio, but what is the most efficient jet engine in existence (currently in use or tested prototype) per category (fan, turbojet, bypass, turboprop, ...)?

Thermal efficiencies are very rarely quoted for aviation gas turbines. The metrics of interest are specific fuel consumption, and power to weight ratio. While a higher thermal efficiency will increase these, SFC and thrust/weight are performance terms that are easier to comprehend, and describe the performance in terms that can directly be used in performance calculations of the aircraft. However, this article from Pratt & Whitney indicates the best aviation gas turbines had just reached about 55% in 2010. But there is no indication of which engine has the highest figure.

Land based gas turbines are more efficient than avaition gas turbines, for a number of reasons. According to this article, the most efficient land based gas turbines, where just under 60%, in 2010. The most efficient installation was a Japanese company, with 59.1% verified on an M701G2 gas turbine at the 1500 MW Tokyo Electric Kawasaki power station in Japan.
Since then GE claimed on the 28th of April 2016, a world record for thermal efficiency, with 62.22%, for a combined cycle plant, in France. GE state that it normally takes about a decade to gain 1% in efficiency. The high thermal efficiency is significantly due to operating at a very high temperature. GE state this plant runs at 2,800 deg. F, but the real number is probably proprietary. They say it's like running the engine at takeoff conditions for its entire flight. As you could imagine, this makes cooling very important. It also takes about 30 minutes to get the turbine from idle to maximum speed. Clearly, that won't work in an aircraft.

This is the GE 9HA gas turbine. You can see the fuel nozzles (one circled) - and that weight does not matter. So, an aviation turbine will never match this level of performance.
In comparison, the first simple cycle model, developed in 1939 is said to have had a thermal efficiency of just 18%.
But maximum efficiency is not always the ultimate goal. A product director from Rolls Royce says:

Everyone would love 60% but the cost is often prohibitive. Not everyone needs a heavy-duty combined-cycle plant. Machines that can achieve baseload within ten minutes such as the Trent 60 are important for the peaking market and a lot cheaper.

Land based combined cycle gas turbines are more efficient than aviation turbines, because;

their design can be optimised with out having to worry about weight. Fuel nozzles in particular are optimised in land based gas turbines.
they can use steam or water inlet fogging (see above!)
they operate at a single rotational speed, so can be optimised for that.

I almost lost interest in this answer as nothing about aviation was to be found until the end - I'd consider moving the part actually answering the question to the top and add the land based part to the bottom (it is interesting, but not directly connected to the question, it gives a hint on the upper ceiling)

@Arsenal Because the question is about something that aviation hardly cares at all. For aircraft, the pure thermal efficiency is of very distant concern. What matters is the overall flight efficiency, which depends on many other parameters, like weight. So question about pure thermal efficiency has no useful answer in aviation context. It's like asking about what car has the lightest drivetrain. Nobody cares, we care about MPG, and heavy hybrids deliver more than weight-optimized transmission. Exactly opposite as in planes. It's reasonable that Penguin focused on area where efficiency matters.

In addition to the three reasons given for land-based combined cycle gas turbines being more efficient than aviation turbines, there is the whole 'combined cycle' thing - the use of a second heat engine (invariably a steam turbine, I believe) to extract power from the gas turbine's exhaust heat.

@Agent_L - the question is quite clear, yeah it might not matter to the average jet engine aviation enthusiast. But inaliahgle is interested in this. If there would be no answer to be found, that's alright, compose an answer out of that and why it doesn't matter. But there is an answer, the latest figure to be found was 55% in 2010, so why not put that first?

@Arsenal. Your comment is a fair one, and I generally prefer and write answers with the bottom line up first, then the background. In this case, it happened the other way around, simply because that's the order of info I found as I researched the topic, and I didn't have time to re-organise. I have fixed it now.

I found a patent on an Internal Combustion Turbine dated around 1929, one of the names was Thyssen, but I've been unable to retrieve the link. They claimed a thermal efficiency of more than 30%. Gesund +


text:
Why do the MiG-15 and MiG-17 have a split air intake in their noses?

MiG-15 and MiG-17 both have a split air intake in their nose. However, they have only one jet engine.
So why is the air intake split? Wouldn't it be a lot less drag to just let the intake flow cleanly into the engine?

Probably because the pilot position occupied the same space as the air intake and it needed to be routed around the pilot and controls.

Also the nose gear bay and retraction system. It might also have a center fuel tank in there too.

I was going to say that a little air conditioning is appreciated by the pilot, but the power-wedgie, not so much. I see that's basically been covered.

The advent of (turbo)jet engines resulted in an important change in design of aircraft- the location of the engine, which can either be mounted on the fuselage  or in the wings (as in Me 262).
Also, the engine was shifted from the front (as in the case of a majority of piston engined fighters) to the rear in jet aircraft (as the exhaust has to be routed below the fuselage otherwise). In this case, the intakes can either be in the nose or in the sides (like in Hawker Hunter).
For the aircraft with nose intakes (a design which was eliminated by the ever increasing size of Radar equipment), the problem was how to get the air to the engine. In case of Mig-15, the Russians split the air in a central splitter, which then passed through two narrow air ducts passing through either side of the cockpit, avionics bay, fuel tanks and the nose landing gear bay. This inlet was quite complex, not to metion it made for a cramped cockpit, but on the other had, it was compact. Similar air intakes were used in aircraft like the Mig-21.
Another method was to put to simply put the cockpit above the air inlet duct, like the F-86 Sabre, which resulted in the raised stance of the cockpit (and better visibility).

The intake is split in order to provide a place for the squishy human pilot to sit. Sure, it would be less drag to have a straight pipe, but that would mean the pilot being subjected to the airflow. If they wanted a straight airflow they would have had to move the pilot position or move the air intake.

A pilot subjected to the airflow would also cause drag. :-)

It would be a drag to the pilot too.

The two air intake ducts of the MiG 15 pass either side of the nosewheel box and the cockpit. The alternative configuration of having a single duct passing underneath the cockpit, as used in the A7 Corsair II, necessitates the nosewheel being mounted further to the rear, which in the MiG 15 with it's short fuselage, would have made the wheelbase too short. Prototypes of the MiG 15 mounted guns in the space between the intake ducts, but this led to engine problems caused by ingestion of gun gas and so the armament was relocated to under the nose.

Just to visualize it:
The following cutaway (source) shows that the split was needed to make space for cabin, avionic, gun and front wheel well:

Aircraft design was just starting to consider compressible air flow. It may have been a proactive plan for supersonic flight (Remember the F-4 intake variable ramps). Tho they were far from trans-sonic flight, it may have been wishful thinking. It would also greatly enhance the rigidity of the airframe considering the cannon it carried.

Hello Jack, welcome to Av.SE. Are these guesses, or do you have authoritative sources to support your statements? Posts made as Answers are expected to provide an answer to the stated question, not just hypotheses.


text:
Why is the Airbus Beluga XL based upon the A330, and not upon the A350 or A380?
Airbus is developing the successor of the Beluga, called Beluga XL.
The original one was based on a standard A300, while the new model will be based on the A330.

Why is the new model based on a quite old design like the A330, and not the more modern A350 (which could generally be more efficient and have more modern avionics)?
In addition, I saw that Boeing built an equivalent aircraft, the Dreamlifter, based upon its biggest model, the 747, so... obvious question: why doesn't Airbus base the Beluga XL off of its giant, the already-tested A380?  This could offer a much better payload capacity, and, maybe, even the ability to carry fuselage sections of future A380s (which, currently, must be transported by trucks due to their size).

The fact it is based upon a A330-200 rather than -300 or any A340 is down to the runway length at Hawarden Airport; I presume similarly the A380 can be ruled out on those grounds. I'd presume also the A350 programme is simply too busy trying to construct customer aircraft to meet existing orders, whereas there's not a several hundred backlog of A330s.

By the way, A330 engines are quite good and lot of spare parts available for the next decades. Avionics can be upgraded (just look at the 747-8, 737-Max and A320Neo. All of them are pretty old sheet design externally, but top notch in terms of avionics. Even the A300-600R, derived from A300-B4 and A310 isn't that old)

Considering that (one of) the main reason(s) for introduction of larger Beluga XL is to meet the demands of the A350 XWB production, it doesn't make sense to have a modified A350 for the next transporter: Airbus would prefer to sell A350s to customers than to modify them for internal use.
A380 is not supported by airports near most of the Airbus production centers- defeating the very purpose of having the Beluga XL fleet.
In the end, it boils down to which aircraft can satisfy two requirements- payload and runway capability at Broughton. From caan.asia/en:

Airbus wants an aircraft which not only meets high payload capabilities - including capacity to carry two fully-fitted A350 wings - but can operate within airfield landing limitations at its UK wing facility at Broughton, to which it will deliver A350 wing covers. Broughton's declared available landing distance for the shorter runway 04 approach is 1,663m (5,460ft).
Airbus considers this restriction to be too tight for a modified A330-300 or A340-500, while the payload requirement is too high for an A300-600. It believes an A330-200 variant - tentatively designated the A330-200XL - could potentially cope with the landing criteria at projected weights of around 135t, and is the most promising option.

Boeing Dreamlifter, on the other hand, is purpose built for B787 transport- wings and fuselage, while the Beluga transports only the wings. The distances involved in the B787 production (compared to the only European production centers of A350 XWB) means that Airbus can afford to transport only the wings, rather than fuselages too.

Please, what are differences between A330-200 (able to operate from Broughton) in comparison to A330-300? Why the second one (that I think is more powerful) does not suit requirements?

In addition, is not true that Beluga XL transports only wings... There are several videos on youtube official channel in which is clearly visible A350 fuselage going out from original smaller Beluga

@LucaDetomi, power does not matter for landing, weight and wing loading do. A330-300 is heavier and has the same wing, so higher wing loading, which leads to higher landing speed and requiring more runway to stop. More than there is.

The runway at Hawarden wouldn't necessarily be a killer for anything needing more runway than an A330-200; one could simply extend the runway (which they were actually planning to do  before Airbus settled on the A330-200).

Another important factor is the engineering cost and time for designing ultimately a handful of Beluga XLs.
The existing Beluga is based off the A300-600. The A330/A340 airframe is fundamentally an updated A300 and are highly similar, including its cross-sectional dimensions.
Therefore, Airbus simply carried over many of the existing Beluga designs and made minor modifications to others. You can see how similar the Beluga XL and the original model look. This is not a new idea, an A340-based Beluga was proposed in the early 2000s with similar shared parts.
Boeing did not have an existing aircraft to work with, so they had to design the Dreamlifter from scratch, as Airbus would have to do with a new A350 or A380-based Beluga.


text:
Why is the speed brake lever not a "spring" one that returns to the default position when released?
I was watching this video in which are described various systems on B737 but at around 6 minutes, it talks about speed brakes and necessity for pilots to maintain their hand on the lever to avoid forget to release them.
It seems perfectly sensible, but I'm asking why this is not a simple spring lever... if is very important that pilot don't forget lever in extended position, and for this reason is necessary that he's maintaining his hand on the lever, why don't make a simple lever that returns to default position after release?
This would sound even more secure... and in both cases pilot hand would be "busy".

Are you asking about the 737 in particular, or about aircraft in general?  Because IIRC all the aircraft I've flown (sailplanes) did have spring-loaded speed brakes/spoilers.

I discovered this feature on the 737 video linked but, sure, I meant about every aircraft. Now that you are saying that in sailplanes it's exactly like I proposed, it means that is not so "strange" as solution. So doubts increases about reasons behind a differen choice...

While I've never flown a 737, I'd guess it's all about complexity and workload.  In a sailplane, you basically have a joystick and the spoiler handle, there's nothing else you need to use your hands for.  In something like the 737, you might need hands for throttles, gear switches, radios, and maybe other stuff that the 737 pilots here could tell you about.  But even going to something with an engine, like my Cherokee, the flap lever has several positions it locks in.  Also, in a sailplane you are more affected by air currents &c, so you're using the spoiler as sort of an anti-throttle...


text:
Why do airliners still have a switch for the non-smoking sign?
I've wondered why airliners have a switch in the cockpit to turn off the non-smoking sign. Why isn't it illuminated all the time, or a better question: Why is it still there, now that everybody should know.

A few airlines still allow smoking on some flights (none in the US though).

to me this is not a duplicate: this question asks why, the other asks if.

I think this is a duplicate as there is information in the answers of the question linked that may answer this question

@Federico And the answers to the other question explain why, as well as if. It's a dupe.

@DavidRicherby but is a secondary answer buried down beneath the accepted one. it is not given, even if this gets closed as duplicate, that would be easily found by a visitor.

@Federico Really, I don't think it's at all hard to find in the answers to the other question. I'd expect anybody who was curious about this question to at least skim all the answers anyway: that's what people who are interested in something do.

Smoking was outright banned on flights in the US in 1998/2000 which was in reality not all that long ago. It should be noted that smoking on domestic flights under 2 hours was banned in 1988 and under 6 hours in 1990. With that in mind we must remember that there are plenty of planes that are still in service and older than that, many were designed long before that time as well. In the view of the airlines its simply cheaper to leave the lights and switches there than put in a new interior panel in many cases.
Also as mentioned in the comments planes are built for the global aircraft market and there are still countries that allow smoking on flights. This question is similar and has some good info as well.
But here in the US the main reason is because the FAA mandates the signs,
For charter (135) operations

§ 135.127 Passenger information requirements and smoking prohibitions.
(a) No person may conduct a scheduled flight on which smoking is
  prohibited by part 252 of this title unless the “No Smoking” passenger
  information signs are lighted during the entire flight, or one or more
  “No Smoking” placards meeting the requirements of § 25.1541 of this
  chapter are posted during the entire flight. If both the lighted signs
  and the placards are used, the signs must remain lighted during the
  entire flight segment.

For scheduled commercial (121) operations

§ 121.317 Passenger information requirements, smoking prohibitions,
  and additional seat belt requirements.
...
(c) No person may operate
  an airplane on a flight on which smoking is prohibited by part 252 of
  this title unless either the “No Smoking” passenger information signs
  are lighted during the entire flight, or one or more “No Smoking”
  placards meeting the requirements of § 25.1541 of this chapter are
  posted during the entire flight segment. If both the lighted signs and
  the placards are used, the signs must remain lighted during the entire
  flight segment.

The reason they are not lighted the entire time most likely stems from history. Since there is a switch to turn them on chances are there is a check list item to flip the switch. You will most likely see them go on and off when the pilot hits that item on the pre flight checklist that may occur at the gate or on the taxiway heading out for take off.
The Piper Archer I fly still has ash trays (there is even one in the pilots seat arm rest)...

Also, it needs a switch simply because you don't want to have things come up on their own immediately when the electric bus is powered. It is a good general practice with electricity to power the line with appliances detached and then connect them one by one that doubly so applies to aircraft systems.

Your link to countries that still allow smoking is 14 years old. I highly doubt it's still accurate.

Dave's answer is largely correct, but it's probably also worth mentioning that many newer airliners have no such switch... because they have no lighted "No Smoking" signs at all. Instead, they use permanent placards.
While the regulations that Dave cited do require the No Smoking signs to be lit for the entire flight if they are present, it also allows permanent placards to be used in lieu of the lighted signs:
14 CFR 121.317(c):

(c) No person may operate an airplane on a flight on which smoking is prohibited by part 252 of this title unless either the “No Smoking” passenger information signs are lighted during the entire flight, or one or more “No Smoking” placards meeting the requirements of § 25.1541 of this chapter are posted during the entire flight segment. If both the lighted signs and the placards are used, the signs must remain lighted during the entire flight segment.

For example, this picture is from the passenger cabin of one of Delta's new 737-900ERs with the Boeing Sky Interior:

Delta Boeing 737-900ER No Smoking Placard
Note that the seat belt sign is still lighted and switched from the flight deck, but the No Smoking sign is just a permanent painted placard.


text:
What was the purpose of the "tail prop" on the Ilyushin Il-62 and why was it necessary?
I read somewhere that the Ilyushin Il-62 had a "tail prop".  What was this device and why was it needed on this aircraft?

I read "prop" as propeller at first.

Are you asking about the rear support wheel? It is for supporting the aircraft and avoiding tilt on the ground.

Image from wikimedia commons. Autor:M Radzi Desa
As the aircraft had four engines in the aft, the cg would've been aft too; so they added a tail prop to prevent tilt on ground.
Interestingly, this doesn't seen to be unique for Il-62. Another aircraft with rear mounted engine, Sud Aviation Caravelle, has something similar to this.

Image from wikimedia commons. Author: Eduard Marmet

in case of the Caravelle it's actually not a support structure but a rear staircase. Early model B727s had one as well.

Looks like a number of tail engines aircrafts like DC 9 and Yak 42 have this 'airstairs'. They double up as tail support while loading, I guess.

they don't. They lack the structural strength and aren't rigidly attached to anything. They're in fact spring mounted to allow for easy manual extension and storage,

@jwenting I'm not sure aeroalias speak of the stairs. On the picture you may see a vertical structure just before the stairs. It may be something behind the aircraft but may also be a support. I really don't know, any reference would be welcome

@ManuH From looking closely at it, including the shadows, it does appear to be attached to the plane, rather than something behind it. My assumption was also that that's what the answer was referring to.

I'm pretty sure a secondary purpose of aiistairs was to support the tail during loading. Back in the day when we used 727's at FedEx the rear stairs were always down during loading & unloading. It doesn't have to support the whole weight of the plane, just whatever amount of differential in CG that was behind the rear wheels until finished loading. The other option, which was used on DC10's was a nose gear tether.

When I look at the shadows on that shot it looks more like it is behind the plane. I can't find any other photos of the Caravelle that show it. Here is a shot of tail stands being used on a couple types of aircraft. The unique thing about the Ilyushin is that it seems to be built into the aircraft itself.

They're called "tail stands" for planes such as the Cessna Caravan.
During loading and unloading the CG can be tail-heavy enough that the aircraft would otherwise fall back on its tail.

I found an Aircraft Operations Manual for IL-62 and IL-62 M of former GDR carrier Interflug (in German language):

http://www.interflug.biz/IL-62%20Flugbetriebsdokumentationen.htm
http://www.interflug.biz/Flugzeuge/IL-62/Handbuecher/FZH_Il62.pdf

According to this manual, the "tail prop" supports several functions.

Supporting parked airplane while centre of gravity isn't controlled: The manual requires to extend the tail prop when parking, before opening any door. Tail prop shall be retracted only after closing all doors and only when centre of gravity is within limits (4.2.5. No. 2., tab. 4.2.5./1).
Taxi with extended tail prop is allowed, speed limit is 25 km/h. (4.2.2. No. 1)
When towing an empty plane the tail prop must be extended, speed limit is 15 km/h. (4.2.2. No. 2)
Moving the plane backward by utilizing reverse thrust: Tail prop must be extended, speed limit is "walking pace". (4.2.2. No. 4)
U-turn on a narrow runway (min. 45 m) by utilizing reverse thrust: Tail prop must be partly extended. (4.2.2. No. 5, fig. 4.2.2./2)

The tail prop features two self-steering wheels and a suspension system. No brakes, no active steering. It is extended and retracted by an electric motor, backed by a hand crank. (4.1.8. No. 1.3, fig. 4.1.8./5)
Probably, the tail prop is also designed to provide some protection in case of a tail strike (since it's wheels partly remain outside fuselage), but this function isn't mentioned in the manual.
Btw., this plane features a 3700 ℓ water tank (just behind the front landing gear bay), for adjusting centre of gravity in some low-load situations. (4.1.24., 4.3.2. No. 2.5, fig. 4.1.2/2 item 5)

Welcome to aviation.SE!

Excellent find and well done in providing excerpts from the docs!

It's a tail stand -- it prevents the airplane from tipping up and back onto its tailfeathers in a most embarrassing fashion, even if you have to load it back to front or unload it front to back (image from Aerospaceweb's question on landing gear):

Improvised tail-props were a feature of early, tricycle landing gear, military aircraft. Grumman's F7F Tigercat, certainly. There are photos with 55 gallon drums + assorted odds and ends as props, in place, and also photos of unsupported planes sitting with nose-wheels off the ground...
At that point in Grumman's history, nobody would have expected resting the tail on the ground would do any damage to the airplane...

This is a very interesting take on CG vs Clift, in this case while the aircraft is not moving!
The wheels are holding the aircraft up.  It would take a relatively small amount of force to tip the plane onto its tail while loading or unloading.  A little like a table with 3 legs instead of 4.
While taking off, the aircraft has to be able to rotate, which means the main gear need to be only slightly aft of the CG when fueled and loaded for flight. On the ground this creates the potential for it to go "tail dragger" if cargo is removed from the front or added to the back.  Moving people from front to back was actually used in airships to help trim CG.
But it is very, very important to realize this is not done "because engines are heavy" and the designed CG is somehow unsafely aft.  Jet engines are actually (lb weight per lb thrust) much lighter than their piston counterparts.  Aircraft with swept wings who must, by necessity, mount the landing gear closer to the fuselage (or in the fuselage) would have a greater tendency for "ground Clift" issues.  The B52 designers solved this by mounting the wing at a higher AOA than the fuselage and taking off WITHOUT rotating, relying only on speed to lift the aircraft off.  This also fits well in pilot training not to "pull up" a large aircraft, as shear mass would make recovery of precious airspeed difficult, even with lots of excess
thrust available.
The "tail pogo" is strictly a safety precaution while passengers, cargo, and fuel are being loaded or unloaded on the ground,

Robert DiGiovanni's response is correct.
While the empty IL62 is a bit tail heavy, once the airplane is laoded with passengers/bags and cargo the CG will be moved forward. Look at how much longer the fwd fuselage is compared to the aft section, its around 4 times longer!
The MD11 has a similar proportion where the forward fuselage is much longer than the front. I've done the load-sheet for MD11 freighters and while the empty CG is VERY tail heavy, with a full load the CG is usually at the fwd limit.

Many smaller turboprops and piston twins use 'tail props' or 'pogo-sticks' as they are known here.  Certainly Twin Otters, BN Islander and Trislanders use them.  Can't comment on the Caravan.  The last one I was son a few weeks ago had floats ;-)

welcome to aviation.se! please consider adding more details, currently you provide interesting information, but you do not actually answer the question "what is its purpose"

authors: αφροδιτη παπαχριστοπουλου


text:
Does high density altitude affect your landing speed?
So I had this question asked in my recent commercial-pilot checkride:
The DPE asked me about high-density-altitude airports; he specifically asked me if I should add airspeed when I'm on landing approach to a high-density-altitude airport.
 My total understanding is that the ground roll and landing roll would be affected, as well as the climb, but I've never heard about landing speed being affected; I've been looking in books and researched on the web but I can't get a solid answer.

Your $v_\mathrm{ref}$ does not depend on density altitude since it is given in Indicated Airspeed, which already accounts for density effects. However, the True Airspeed and therefore also Groundspeed will be higher at a higher density altitude, resulting in more runway required to stop.
The Flight Safety Foundation has a nice summary of factors influencing $v_\mathrm{ref}$:

Factors Affecting the Final Approach Speed
The following airspeed corrections usually are not cummulative;
  only the highest airspeed correction should be added to VREF
  (unless otherwise stated in the AOM/QRH):

Airspeed correction for wind;
Airspeed correction for ice accretion;
Airspeed correction for autothrottle speed mode or autoland; or,
Airspeed correction for forecast turbulence/wind shear conditions.

Gross Weight
Because VREF is derived from the stall speed, the VREF value
  depends directly on aircraft gross weight.
The AOM/QRH usually provides VREF values as a function of
  gross weight in a table or graphical format for normal landings
  and for overweight landings.

(FSF ALAR Briefing Note 8.2 - The Final Approach Speed)
Altitude, Density Altitude or Pressure are not listed as factors affecting $v_\mathrm{ref}$.


text:
How does one interpret the satellite data on flight MH370?
There seems to be a problem with some of the data released by the satellite company Inmarsat.
Specifically, the handshake at 00:10:58 on 7/03/2014.
In the left column the difference in time-stamp between the ground station initiating the request and the receipt of the return signal from the aircraft is given as 1.928 seconds. However, the prior handshakes at 19:41, 20:41, 21:41, and 22:41 took 1.996, 1.997, 1.998, and 1.999 seconds.
This would imply that the round trip distance between the ground station and the aircraft changed by about 13,000 miles in 1.5 hours.  What gives?

TL;DR.

Use the BTO not the difference in time-stamps for request & acknowledge.
BTO is an offset in μs, not a round-trip time.
There's a mean bias value (-495,679 μs) that includes the signal processing delays in equipment.
The distance is ground-station to satellite to aircraft to satellite to ground-station.

Inmarsat data
As I understand it, to calculate distance you should be starting with the Burst Timing Offset (BTO) not the difference between time-stamps in the Inmarsat data. There are latencies in various stages in the electronic processing of messages, not all of these are distance related.
Time    BTO(μS)
19:41   11500
20:41   11740
21:41   12780
22:41   14540
00:10   18040

There's an example on page 54 of MH370 - Definition of Underwater Search Areas:

The aircraft AND the satellite are in motion in three dimensions
The satellite is at an altitude of around 35,811 km above the earth's surface, the satellite is moving cyclically north and south of the equator and the aircraft is flying over a roughly oblate spheroid so changes in it's distance from the satellite are not a simple function of changes in the aircraft position relative to the surface.

Not to scale. E&OE. etc

References

Inmarsat 3-F1 satellite position
Inmarsat 3-F1 satellite tracking
Inmarsat raw log data for MH370
MH370 - Definition of Underwater Search Areas

The "R" terms given in the equation (in reply to my question) do not seem to represent the actual distances.For example, using the coordinates 0 degrees N/101.7 Degrees E the distance from the orbiiting satellite to these coordinates is 23,182 miles. And from the satellite to Perth, Australia is another 24,250 miles.  But the maximum value for the term to the right of the equal sign is (.014810xC)/2=1379 miles. And since the distance from the orbiting satellite to Perth is much greater than 1379, this would make the the range value (from the aircraft to the satellite) negative.

@John: See P54-55 of the second link. The bias value is negative.

I have updated the answer to include the relevant section of the ATSB report that gives examples of the calculations of aircraft range from Inmarsat timing data.


text:
How unique are waypoint names supposed to be?
I had thought that someone such as ICAO was coordinating the assignment of five-letter waypoint names such that they were globally unique.
Now I notice that OpenNav knows three different waypoints named PINTO, two of which are on the same continent.
Is this supposed to happen? Are there rules for how close identically-named waypoints can be to each other?
(A significant factor in the AA 965 crash was that there were two NDBs with the same ID close to each other, and the FMC selected the wrong one to fly towards. This might be inevitable when NDBs usually have one- or two-letter IDs, but surely with a big fat five-letter namespace uniqueness ought to be achievable?)

Related: How are airspace fixes named?

@farhan: I edited the question's title back to what I actually want an answer for.

@HenningMakholm Not a problem. I removed the word unique because there cannot be levels to uniqueness. E.g. something cannot be 10% unique or 58% unique.

@Farhan: But something can be "globally unique" or "unique within a hemisphere/continent/1000 nm distance", or some other quantified amount of uniqueness.

You might enjoy this

@Farhan I was going to say the same thing. "How unique" doesn't make any sense. People are always saying "very unique," "really unique," etc., these days, none of which makes any sense. Something either is unique or it's not. It makes sense only to ask "Does X have to be unique?" or to say "Something is unique."

@birdus: So what is your response to my explanation of how something can indeed be more or less unique?

The names are not globally unique and when entering the plan into the FMC, the crew should check the leg distance.  They are sufficiently far enough that the crew will see 1,812 miles instead of 135 miles.
Many FMCs will also only offer "sensible" choices and a deliberate action is required to select a "nonsensical" waypoint.
Often, a mistake in choosing the wrong waypoint will also alert them with a "not enough fuel" message, or an obvious gross error such as a total distance of 3,323 miles instead of the 756 miles expected.

So, a waypoint name along with its distance makes its unique.

Unfortunately, according to the Wiki article about flight AA965 that was linked in the question, it sounds like R was used for both the ROMEO and ROZO NDBs in Columbia. Since Columbia is only but so big, and these pilots were in a pressure situation, they failed to note the difference.

@Farhan not if you're equidistant from the waypoints!

@rbp Yes, but there should be no sensible plan where sequential waypoints are as far apart as the half-distance between same-named fixes.  If you're over Honduras, your next fix isn't going to be in Idaho, nor in central Chile.

That's a Better answer

@J...: The point is that -- if these answers can be trusted -- there seems to be no rules (or even loose guidelines or rough statements of intent) for what "the half-distance between same-named fixes" can be, so how can you make sure of that?

@HenningMakholm I'm not sure if that's a rhetorical question.  I guess the only way to be sure would be to run the data through analysis and check.  If there are any examples you can find where a real flight path would encounter ambiguity between two waypoints with the same name it would be interesting to see it. If you consider the longest leg between waypoints on any flight plan, it's not going to be a very large distance.  Within that radius, I'm not sure you'd find two same-named waypoints anywhere.

Each country's aviation authority is responsible for naming their own waypoints, so it's up to them how far they try to avoid duplication. In the US, the FAA's Procedures for Handling Airspace Matters section 3-3-4 simply says:

e. AIM [Aeronautical Information Management] must not duplicate any radio fix, waypoint, marker beacons or
  compass locators names.

So they are (or should be) unique within the US, but they aren't guaranteed to be unique globally, although presumably other countries also have policies to avoid duplicate names. For example, this answer quotes similar language from the UK's CAA and has some additional information.

So if country A and country B both decide to define MYPNT a few kilometers from each other across their mutual border, neither of them will have done anything wrong?

@HenningMakholm No.  But they just wouldn't do that.  Just because the law doesn't prevent it doesn't make it right or sensible!

@HenningMakholm They won't have done anything illegal, but they will have done something stupid and potentially dangerous. I can't find any documents on it, but I guess that most national aviation authorities are smart enough to avoid doing that, and it's even possible that there's some kind of international co-operation on naming between individual countries. In Europe ATC (Eurocontrol) is kind of international anyway, so there's probably some coordination there at least.

According to ICAO document Annex 11 — Air Traffic Services; Appendix 2:

2. Designators for significant points marked by the site of
  a radio navigation aid
(...)
2.1.2 In selecting a name for the significant point, care shall be taken to ensure that the following conditions are met:
a) the name shall not create difficulties in pronunciation for pilots or ATS personnel when speaking in the language used in ATS communications. Where the name of a geographical location in the national language selected for designating a significant point gives rise to difficulties in pronunciation, an abbreviated or contracted version of this name, which retains as much of its geographical significance as possible, shall be selected;
Example: FUERSTENFELDBRUCK = FURSTY
b) the name shall be easily recognizable in voice communications and shall be free of ambiguity with those of other significant points in the same general area. In addition, the name shall not create confusion with respect to other communications exchanged between air traffic services and pilots;
(...)
3 Designators for significant points not marked by the site of
  a radio navigation aid
3.1 Where a significant point is required at a position not marked by the site of a radio navigation aid, the significant point shall be designated by a unique five-letter pronounceable “name-code”. This name-code designator then serves as the name as well as the coded designator of the significant point.
3.2 This name-code designator shall be selected so as to avoid any difficulties in pronunciation by pilots or ATS personnel when speaking in the language used in ATS communications.
Examples: ADOLA, KODAP
3.3 The name-code designator shall be easily recognizable in voice communications and shall be free of ambiguity with those used for other significant points in the same general area.
3.4 The name-code designator assigned to a significant point shall not be assigned to any other significant point.


text:
What determines crosswind limits?
What is typically the limiting factor for how large a crosswind component a given plane can be safely landed in?
My impression is that larger planes are less sensitive to crosswind than smaller ones -- for example, many commercial airports seem to be doing quite well with a single or multiple parallel runways for transport-category aircraft, whereas airports catering to light aircraft will be proportionally more likely to have (and routinely use) a system of intersecting runways.
I can imagine one reason for this, namely that a plane with a higher landing speed will need less of a crab angle for a given crosswind component. But is this actually the determining factor?
If yes, then is is safe/legal/practical to land a light aircraft in more crosswind than it can usually handle, by landing faster, assuming there's a nice long runway available to decelerate on? (In other words, can an airport make up for having no crosswind runway by having a longer primary runway instead?).

This may help understand the difficulty. There is no limit while landing manually, but automated landings (Cat III for instance) have imposed limits. Usually the company requires pilots to remain within crosswinds demonstrated by the manufacturer for certification.

Instead of a hard limit, there usually is a maximum demonstrated crosswind component. The question is also related

I am aware that the limit is not hard, but there are situations where a PIC will decide "I won't try to land here because of the crosswind" and divert to somewhere that has a more suitably aligned runway, right? I'm interested in the factors that affect that decision, and why it seems to be necessary less often in airliners than in light aircraft.

The ability of the pilot's feet to reach the rudder pedals.

The limited deflection of the rudder will limit the ability of the pilot to forward slip and de-crab. But the most important factor maybe the undercarriage capability to sustain lateral constaint.

Your question is not easily answered, at least in my opinion, but I'll have a go at it.
Your impression that larger airplanes are less sensitive to xwinds than smaller airplanes is correct, very much so in my experience. I lack the expertise to give you the aerodynamic basis, the equations if you will, but I'm fairly certain that saying an aircraft's sensitivity to a xwind is inversely proportional to it's mass, landing speed, and wing loading is a correct statement. As I remember, the max demonstrated xwind for a Cessna 172 is 15 kts, for a 747 either 25 or 30 kts, and I would much rather be landing a 747 in a 30 kt xwind than a 172 in a 15 kt xwind. In 747 sims we would occasionally crank the xwind up to 40 kts for an interesting but still safe landing.
Using extra speed in a small aircraft to help is not, in my opinion, a good idea. You'd certainly want to add a bit to your approach speed to handle gusting conditions (which typically accompany crosswinds), but what you need to do is transition from flying speed to non-flying speed as quickly as possible rather than extending the time of that transition. You need to get the weight off the wings and on to the wheels as quickly as possible.
There is one exception to what I just said. Back in the 1950s when I first learned to fly, they used to teach what they called 'wheel landings' for handling crosswinds in taildraggers. The problem with a taildragger is that the c.g. is behind the main gear. If you touch down in any kind of a crab, it wants to switch ends. In other words, a ground-loop. The idea of a 'wheel landing' was to get the main wheels on the runway with extra speed (after having slipped to the touchdown) so that the rudder (with the tail still off the ground) had plenty of authority to counteract weathervaning, and of course with the upwind aileron fully up. Then you were supposed to keep the airplane straight while the your speed dissipated to where the tail could no longer fly. I have no idea if they still teach that technique.
So now you're the PIC and having to decide whether to land in a xwind. Following are some of the things I can think of that might enter into your decision making:

What's your personal confidence level? If you're nervous about the situation you're probably not going to do as good a job as if you were relaxed.
What's your experience level in the airplane? Aircraft, even of the same weight class, vary in their handling qualities in a xwind, and the more you've flown them in xwinds, the better you're going to be in that model of aircraft in a xwind.
High wing or low wing? The closer to the ground you get the more the wind decreases, and that few feet of difference can make a difference. Also, I suspect, but do not know, that having most of the wing below the c.g. rather than above the c.g. helps.
What is the runway surface, and what is its condition? In other words, how slippery is it going to be. A dry paved runway is, of course, the best. However, a dry paved runway with a lot of reverted rubber in 120 F (49 C) temperatures can be interesting. Ice or wet grass is perhaps the worst. I put a Cessna 150 off the runway in wet grass back in the 1970s.
How bad do you need to land there? Some might say that doubts about being able to safely land should always override the need to land, but that's not the real world.
For a light aircraft, you may be able to turn that strong xwind into a strong headwind by using a taxiway. I've done that at a uncontrolled field.

A word of caution: Just because the xwind is within the max demonstrated xwind component does not necessarily mean you can safely land, it just means an experienced test pilot did it on a reasonable runway surface. The max demonstrated xwind is a guideline that is sometimes safe to exceed, sometimes not, and sometimes not safe to even get close to.

Agreed... plus add in recency of experience, if you fly once every month, you're going to be less likely to go to the limits, versus someone who flies daily and knows exactly where their limits are, and how to fly at them. Also, the wind sensors and winds constantly are changing, what worked a minute ago, might not be safe to land in now.

Thanks for the practical perspective. When I speculated about landing faster I had in mind something to increase the stall speed (such as refraining to extend flaps) so you'd be in "non-flying speed" appropriately faster. But I understand from your explanation that this just isn't done.

@HenningMakholm Apologies for not having picked up on your intention. You are correct, though, that in a strong xwind it is generally advised to use minimal flaps in some aircraft as you will be 'slipping' into the xwind. As I remember (increasingly a problem at my age) there used to be an admonition in the Cessna 172 POH against slipping with more than 30 degrees of flaps back when you could go to 40 degrees in a 172. I think newer 172s have 30 degrees as their max extension.

For Part 23 airplanes, the answer is:
FAR §23.233:

Directional stability and control.
(a) A 90 degree cross-component of wind velocity, demonstrated to be safe for taxiing, takeoff, and landing must be established and must be not less than 0.2 VSO.


text:
How much thrust is available from the Allison T-56-A-15 engine?
What is the thrust availability per engine in Lockheed C130H?
The engines are Allison T-56-A-15 (SHP=4100, SFC=0.5)
Propellers = NP2000 eight Bladed(Prop Efficiency if assumed to be 90% constant at all speeds)
SHP - Shaft Horse Power

@mins That answer provides the KW output for the engine, which isn't the same as the thrust it provides which is entirely dependent on the propeller efficiency. The thrust should be measured in newtons, kg-f, or lb-f.

@RonBeyer: That's right, it's the power of the turboprop, the thrust developed by the propeller with this input power will depends on the speed, and the propeller efficiency (which is not constant over the speed range ➭ Fn = SHP × 375 × efficiency / speed in MPH).

What is the "speed" in your equation? The speed of the aircraft? The formula breaks down at zero speed (obviously), and speeds approaching zero can't have infinite thrust. Is the "speed" value a cruise speed? At least in another field (marine) we measure thrust using bollard pull techniques, which is akin to chaining the boat to the dock and increasing power until the thrust produced maxes out...

@mins The 'speed' in the equation, is it cruise speed?

@Evilz: Yes, it is.

@RonBeyer: At 0 speed, there is no work, equivalent thrust is null too except the core thrust that may be 15% of the total, and that isn't taken into account here. I think this formula is an approximation for comparison turbojet vs propeller purpose. See more.

@mins Are the effects due to propeller geometry(dimensions, twist, no. of blades etc.) included in the propeller efficiency? If no, then what role do they play in the availability of thrust?

@Evilz: I think you should ask a separate question, as this is not a forum site (see Tour, or chat in a chat room). Efficiency takes all that into account, but remember that efficiency varies with speed and density, etc, and also blade pitch (if pitch is variable). Another issue is that you need to know how much horsepower is produced by the engine when the a/c cruises (this is likely not the maximum hp).

@mins Thanks, I'll keep that in mind.

Your question is hard to follow. You may edit it with new lines when suited, space befor opening paranthesis, where you find those figure, the signification of those figures (a.g. what does SHP means?),...

From Raymer's "Conceptual Aircraft Design", page 327, equation 13.15:
$$ \eta_p = \frac{TV}{P}$$
Where:

$\eta_P$ is the propeller efficiency
$T$ is the Thrust [kN]
$V$ is the True Airspeed [m/s]
$P$ is the engine power [kW]

Reverting the formula you get that the Thrust per engine is given by:
$$ T  = \frac{P \eta_p}{V}$$


text:
Do redundant angle of attack and sideslip sensors use a voting mechanism, or do they display data directly?
In civil transport aircrafts (featured with dual/triple redundancy), how does airplane's Airflow angle (AoA/AoSS) sensor measurements read by the pilot? By using voter mechanism as used for airspeeds or individual sensor measurements to pilot and co-pilot?

AoA display is usually an option or not available at all. See also this question for the optional AoA display in the 737.

Airspeeds don't use voting for display in any aircraft I can think of. It is always one sensor to one display for all sensors. Voting is only used for excluding inputs from auto-flight systems and flagging the display as likely errorneous, but pilots generally still have to select which sensor they want to trust.

The A320 is fitted with an air data inertial reference system (ADIRS) made of 3 ADIRUs. ADIRU 1 data (altitude, speed, etc) are shown on Capt side, ADIRU 2 on F/O side. ADIRU 3 can be selected manually.

Each ADIRU contains an air data reference (ADR) and an inertial reference (IR). Each ADR receives the AoA value from a separate AoA probe.

The AoA value is not displayed by default, but it is accessible on the MCDU after selecting the related page. The values are always monitored for validity by FAC/ELAC before being used.
The final value is the mean of valid inputs. If one ADR disagree it is silently discarded. If all ADR disagree, a message is displayed. Any ADR can be shut down using the overhead panel. From this report:

If one ADR has been detected as being faulty, or has been rejected by the flight control computers, and there is an airspeed or angle of attack disagreement between the remaining two ADRs, then a level 2 alert is raised. This activates the master caution and presents an amber NAV ADR DISAGREE message on the ECAM.

The way the discrepancies are managed can lead to rejecting a valid value because the two other ADR are wrong, but agree.

All pictures are from the report.

Do you know if AoSS (sideslip angle) is shown anywhere in the A320? The sideslip indicator / beta target on the PFD is obviously related, but it doesn't show the angle in degrees.

@Bianfable: It's a measure in g. From the FCOM about the sideslip index: "in flight it shows sideslip (as furnished by ADIRS). One centimeter of displacement indicates 0.2g. The sideslip index is against its stop at 0.3g". So I assume the ADIRS uses the IRS to detect the slip, it's not a static pressure difference.

I'd also like to note, that there are several ways that various plane manufacturers handle output. 1) Show them separately, as stated in this answer 2) Take the average (as long as the difference is within tolerance) 3) Output only one, using the other to validate and switch over as backup. Likewise, there are several methods for fault detection.

@MishaP: That's a good addition, and you could make an answer, that would be more complete than this one.


text:
What does "Constant wind condition during flight" mean?
What is constant wind condition during flight? Is it zero wind velocity at an instant (say at level flight) or wind with constant speed, say 20 m/sec (at steady level flight or whatever it is) Which statement is correct?

It means constant, or unchanging wind magnitude and direction.  It could be still air or just a constant upper level wind.  That can happen on local flights but is rare on long cross country flights.